A first balancing bench is known that comprises first and second fastener tools that are set into rotation by a conventional motion generator, e.g. an electric motor.
The operator then places the transmission shaft on the balancing bench by fastening its free ends respectively to the first and second fastener tools, e.g. by using bolts.
When the transmission shaft is in position, the first and second fastener tools are driven in rotation at the required speed.
Conventional balancing means, e.g. including piezoelectric sensors and making use of mathematical principles relating to the Fresnel diagram, then inform the operator when it is necessary to balance the transmission shaft, while also specifying the modifications that need to be implemented.
If modification is required, the operator will generally retouch a portion of the transmission shaft that is provided for this purpose, by removing or adding material so as to enable the center of gravity of the transmission shaft to be positioned on its axis of rotation.
Nevertheless, in order to ensure that balancing is effective and entirely reproducible, it is essential for the transmission shaft to be accurately positioned on the balancing bench.
It will be understood that if it is not accurately positioned, the result obtained will not be optimized. If the transmission shaft is not in a position that is strictly identical to the future position of the transmission shaft, e.g. on board a rotorcraft, then once it has been installed on board the rotorcraft, the transmission shaft will end up by not being balanced.
The balancing bench enables a transmission shaft that is placed in a given position to be balanced, with the balancing means not adapting its data as a function of said position. Any positioning error can therefore lead to catastrophic consequences, since the transmission shaft cannot be properly balanced.
Unfortunately, in order to avoid generating vibration in a rotorcraft, balancing must be extremely accurate, with the center of gravity of the transmission shaft moving through no more than a distance of about 0.015 millimeters (mm) at the most during the balancing procedure.
Consequently, it is impossible to obtain sufficient accuracy with the prior art device having fastener tools that are connected to a transmission shaft by means of bolts.
In addition, even if it is possible to envisage positioning the transmission shaft correctly, that would necessarily be to the detriment of the time taken by the operator to perform that operation, since the operator needs to position the transmission shaft manually with very great care. The profitability of such a device would therefore be greatly reduced insofar as it would take a particularly long time to balance a single transmission shaft.
In addition, it is also essential that the balancing bench is itself balanced, or at least that its fastener tools are. Since the fastener tools are themselves fastened to the transmission shaft, the balancing bench ends up balancing the assembly comprising the fastener tools and the transmission shaft.
Consequently, if the fastener tools are not properly balanced to begin with, then the transmission shaft will not be properly balanced, with any balancing defect in the fastener tools naturally having repercussions on the transmission shaft.
The center of gravity of each fastener tool must therefore lie on its axis of rotation, and the first and second fastener tools must have the same axis of rotation.
To summarize, balancing cannot be performed correctly unless firstly the first and second fastener tools are themselves perfectly balanced and possess a common axis of rotation before the transmission shaft is installed on the balancing bench, and unless secondly the transmission shaft is fastened to the first and second fastener tools in a position that is identical to the position it will have in the rotorcraft.
A second balancing bench is also known that makes use of fastener tools that are arranged inside the free ends of the transmission shaft, and that makes use of fluid bearings.
The fastener tools then present outside diameters that are smaller than the inside diameters of the transmission shaft so as to be capable of being placed inside the transmission shaft. The difference in diameter is then small, on the order of 0.015 mm.
As a general, rule, a fluid bearing consists in forming a film of fluid, of oil or of air, in the space between the outside diameters of the fastener tools and the inside diameters of the transmission shaft. The fluid thus enables each fastener tool to be centered inside the transmission shaft. The transmission shaft is thus correctly and accurately positioned.
In addition, friction between the transmission shaft and the fastener tools is then zero, so it is easy to set the transmission shaft into rotation with the help of jets of air blown against the outside surface of the transmission shaft, e.g. using two diametrically-opposite jets.
Generally, that device is most effective. Nevertheless, when the ratio of the bearing surface divided by the diameter of the free ends of the transmission shaft that is to receive the fastener tool is small, where the bearing surface corresponds to the length of the free end that is suitable for being carried by the fastener tool, then the device is observed to jam.
For example, when said ratio is less than 5%, the fluid bearing does not enable the transmission shaft to be properly centered. At high speed, the transmission shaft then often takes up a wrong position because the fluid film breaks and the shaft jams, thereby preventing balancing from being performed.
In addition, it is difficult to arrange the transmission shaft on the balancing bench since the fluid bearing needs to be sealed, for example. Consequently, it is found to be lengthy and thus expensive in terms of manpower, and also to be relatively fragile.